linux-kernel - Re: [PATCH v5 11/12] x86/paravirt: switch functions with custom code to ALTERNATIVE

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Message-ID: <20210308183058.GC12548@zn.tnic>
Date:   Mon, 8 Mar 2021 19:30:58 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>
Subject: Re: [PATCH v5 11/12] x86/paravirt: switch functions with custom code
 to ALTERNATIVE

On Mon, Mar 08, 2021 at 01:28:43PM +0100, Juergen Gross wrote:
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index 36cd71fa097f..04b3067f31b5 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -137,7 +137,8 @@ static inline void write_cr0(unsigned long x)
>  
>  static inline unsigned long read_cr2(void)
>  {
> -	return PVOP_CALLEE0(unsigned long, mmu.read_cr2);
> +	return PVOP_ALT_CALLEE0(unsigned long, mmu.read_cr2,
> +				"mov %%cr2, %%rax;", ~X86_FEATURE_XENPV);

Just some cursory poking first - indepth review later.

Do I see this correctly that the negated feature can be expressed with, to use
this example here:

	ALTERNATIVE_TERNARY(mmu.read_cr2, X86_FEATURE_XENPV, "", "mov %%cr2, %%rax;");

?

And then you don't need to touch the patching code for ~feature handling
and the flags byte.

If you want it syntactically sugared, you can define a separate
ALTERNATIVE_NOT macro using ALTERNATIVE_TERNARY...

Hmmm.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette